Advanced multi-bit magnetic random access memory device

ABSTRACT

An advanced multi-bit magnetic random access memory device and a method for writing to the advanced multi-bit magnetic random access memory device. The magnetic memory includes one or more pair-cells. A pair-cell is two memory cells. Each memory cell has a magnetic multilayer structure. The structure includes
         a magnetically changeable ferromagnetic layer, a ferromagnetic reference layer having a non-changeable magnetization state, and a corresponding spacer layer separating the ferromagnetic layers. The memory cells are arranged such that an effective remnant magnetization of each of the cells is non-parallel from the cells&#39; long-axis. This allows for more than one-bit to be stored as well as for efficient writing and reduced power consumption.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 11/117,453, filed Apr. 29,2005, now U.S. Pat. No. 7,336,528 which claims priority from KoreanPatent Application No. 2004-0086392, filed Oct. 27, 2004. The entiredisclosures of the prior applications, application Ser. No. 11/117,453and Korean Patent Application No. 2004-0086392, are incorporated hereinby reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to magnetic random access memory, and moreparticularly to an advanced multi-bit magnetic random access memorydevice and a method for writing to the advanced multi-bit magneticrandom access memory device.

2. Description of the Related Art

Rapid growth in digital information generation is likely to face a newwave of explosion in high density memory demand due to the growingpopularity of mobile devices. Magnetic random access memory (MRAM) isone of the most promising candidates to provide energy efficient andnon-volatile memories.

The most attractive advantages of MRAM are that the mobile deviceretains the current state of work when powered down, and, additionally,longer battery run-times can be provided for mobile devices fromnotebook computer to cell-phones, since non-volatility enables MRAMchips to consume less power than conventional Dynamic Random AccessMemory (DRAM) and Static Random Access Memory (SRAM). MRAM operates onthe principle of storing data bits using magnetic charges instead ofelectrical charges as used by DRAM and SRAM.

In the related art, an MRAM is known where one bit of information isstored in a magnetic multilayer cell, which comprises both magnetic andnon-magnetic layers. As shown in FIG. 10, in its simplest form, amagnetic multilayer memory cell 10 includes four layers. These layersinclude an antiferromagnetic layer 11, and two ferromagnetic layers 12,13 separated by a non-magnetic spacer layer 14.

The non-magnetic spacer layer 14 may either be conductive ornon-conductive. In a memory having a conductive spacer, a spin-valveelement is created, while a memory having a non-conductive spacer layerproduces a magnetic tunnel junction (MTJ) magnetoresistive element.

One of the ferromagnetic layers 13 is exchange-coupled to theantiferromagnetic layer 11, effectively pinning the orientation ofmagnetization in the ferromagnetic layer 13. This ferromagnetic layer 13is known as the pinned layer.

When an external magnetic field is applied to such a cell, themagnetization of the other ferromagnetic layer 12, the so-called freelayer (the non-pinned layer), will react to the field.

Sensing or reading the information stored in the cell is based on amagnetoresistance effect.

The electrical resistance of such a multilayer cell 10 depends on therelative orientation of the magnetization in the two ferromagneticlayers 12, 13. In this case, there is low resistance when bothmagnetization orientations are in parallel (P) alignment and highresistance when they are in antiparallel (AP) alignment.

The building block of the MRAM architecture is its crossed-wiresstructure, where a magnetic element is located at the intersection pointof two orthogonal wires. The writing or addressing process of an MRAMcell is accomplished by applying two orthogonal magnetic field pulses,generated by sending an electrical current, down the wires. The twowires are often referred to as the word line (Hy field, along the shortaxis of the magnetic element) and the bit line (Hx field, along the longaxis (or easy axis) of the magnetic element).

The strength of the magnetic fields applied are such that one fieldalone cannot switch the magnetization of a magnetic element, but ratherit requires the combination of both magnetic fields from the word andbit lines, for cell selectivity. However, the present cell writingtechnology relies on applying a long magnetic field pulse, tens ofnanoseconds, long enough for the magnetization to reach the finalequilibrium state. This method of writing is also known as thequasistatic writing which is not only slow, but also not energyefficient.

Also, in such a memory having a multilayer cell structure, there is alimitation of memory density. In most cases, only one bit of informationcan be stored at the cross-point (the intersection point of the twoorthogonal wires) in the MRAM architecture.

Recent attempts have been made to overcome this limitation of memorydensity. For example, U.S. Patent Publication 2003/0209769 to Nickel etal. discloses an MRAM device having a multi-bit memory cell. Inparticular, each memory cell includes two magnetoresistive devicesconnected in series.

Each of the two magneto-resistive devices has sense layers withdistinctly different coercivities and, therefore, requires differentwriting currents. Thus, write operations can be performed selectively onthe two magneto-resistive devices. Each multi-bit memory cell has fourlogic states with different resistance values in each state. This allowsfor storage of more data than a related art single-bit memory cell,which has only two logic states.

U.S. Pat. No. 5,930,164 to Zhu also discloses a magnetic memory devicehaving four logic states and an operating method thereof. The memorydevice of Zhu includes a multi-bit system accomplished by stacking twoor more memory cells on top of each other separated by a thickconductive layer to prevent magnetic coupling between memory cells.

In Zhu, each memory cell comprises a free magnetic data layer to storeone bit of information, a hard magnetic layer as a reference layer, anda barrier layer between the free and hard magnetic layer. Essentially,each memory cell is an independent MTJ device. The memory cells havedistinct coercivities, therefore allowing independent cell writing byone or two magnetic fields. For sensing or reading process, these memorycells have different Magnetoresistance (MR) ratios, therefore creatingfour resistance states for sensing of multi-bit information. The writingmethod, however, relies on a quasistatic writing method which is notonly slow, but also not energy efficient.

U.S. Pat. No. 5,703,805 to Tehrani et al. discloses a method fordetecting information stored in an MRAM cell having two magnetic layerswith different thicknesses.

In the Tehrani et al. device, there are two magnetic data layersseparated by a non-magnetic spacer. Each magnetic data layer can storeone bit of information. A writing process for this device uses differingcoercive forces for the two magnetic data layers, hence, independentwriting is possible using a quasistatic method. A quasistatic writingmethod relies on applying a long magnetic field pulse of severalnanoseconds that is long enough for the magnetization of a cell to reacha final equilibrium state. As noted above, this method of switching isnot only slow, but also not energy efficient.

Further, the reading process of the Tehrani et al. device is verycomplex since six magnetic fields are required to determine the state ineach data layer separately. This creates a slow and inefficient readingprocess.

Finally, U.S. Pat. No. 6,590,806 to Bhattacharyya discloses a devicehaving two magnetic data layers separated by a pair ofantiferromagnetically coupled magnetic layers. The two free magneticdata layers have distinct coercivities and hence, independent writing ispossible.

The Bhattacharyya device has four resistance states depending on themagnetic configuration of the free data layers to theantiferromagnetically coupled layers. Therefore, the reference layer forthe magnetoresistance effect are the pair of antiferromagnetic coupledlayers. This means that if this device is to be used for additional datalayers, each data layer requires one pair of antiferromagnetic layers,providing for a complicated device structure. Also, the device relies onthe conventional quasistatic writing method.

Further, the manufacturing process of related art MRAM devices has onlybeen known to produce low yields of the memory due to high sensitivityto cell defects, which causes domain nucleation, and hence, wideswitching field distribution.

SUMMARY OF THE INVENTION

Illustrative, non-limiting embodiments of the present invention overcomethe above disadvantages and other disadvantages not described above.Also, the present invention is not required to overcome thedisadvantages described above, and an illustrative, non-limitingembodiment of the present invention may not overcome any of the problemsdescribed above.

According to an aspect of the present invention, an advanced multi-bitmagnetic random access memory and a method for writing to the multi-bitmagnetic random access memory that improves the memory density of MRAM,and also simplifies the memory fabrication process by reducing thenumber of electrical current lines.

According to another aspect of the present invention, a multi-bit MRAMdevice is provided which stores more than one bit of information at oneintersection between the bit and word lines. In a non-limitingembodiment, two independent information bits are stored in a pair ofmemory cells (a pair-cell) where each memory cell holds a single bit ofbinary information. Stacking procedures leading to four and eight bitscells are also disclosed. Of course, one of ordinary skill in the artwill recognize that similar multiple bit cells may be realized using thesame principles as for the two, four, and eight-bit cells.

More particularly, in an exemplary embodiment of the present invention,the magnetic memory comprises one or more pair-cells, each pair-cellcomprising two memory cells, and each memory cell having a magneticmultilayer structure. The multilayer structure of each memory cellcomprises a magnetically changeable ferromagnetic layer, a ferromagneticreference layer having a non-changeable magnetization state, and acorresponding spacer layer separating the ferromagnetic layers. Further,the memory cells are arranged or shaped such that an effective remnantmagnetization of each of the cells is non-parallel from the long axis ofthe cells (i.e., the remnant magnetization of each of the cells is notparallel to one other).

According to another aspect of the present invention, a memory isprovided that allows for fast and efficient switching. This may beaccomplished by spin precessional switching. Except for high operationfrequencies, spin precessional switching is energy efficient because itrequires a lower switching field as compared to the aforementionedquasistatic writing process. Shorter field pulses further reduce powerconsumption.

The writing scheme in one of the embodiments of the invention uses thespin precessional nature of ferromagnetic material in which an easy axismagnetic field (+Hx or −Hx) used to define the final state of the memory(1 or 0) is combined with an orthogonal hard axis excitation field (+Hyor −Hy) to initiate spin precession. In this switching process, the Hxor Hy field alone is not able to switch the magnetization. Rather, thecombination of the Hx and Hy fields is the essence of selective writingin the memory array. Spin precessional switching as used in theinvention preferably uses a fast rising Hy field, in the order of 0.5 nsor less.

In an exemplary embodiment of the invention, the Hy excitation fieldpulse is required to have a very short rise time (<0.5 ns) and pulseduration is of the half the magnetic spin precession period.

The pair of memory cells can either have individual CMOS or shared CMOSfor separated reading of stored information.

Ultra-fast magnetization switching dynamics is governed by the dampedprecession of magnetization about an effective magnetic field, H_(eff).The effective magnetic field H_(eff) is the sum of the applied fieldsand the magnetic anisotropy field of the memory cell which is dominatedby the shape anisotropy.

The motion of magnetization M is described by theLandau-Liftshitz-Gilbert (LLG) equation,dM/dt=−γ(M×H_(eff))+(α/Ms)(M×dM/dt), where M is the magnetizationvector, γ is the gyro-magnetic constant, α is the phenomenologicalGilbert damping constant, and Ms is the saturation magnetization. In thecase of MRAM, the effective applied magnetic field is the vector sum ofthe Hx and Hy.

A fast rising magnetic field pulse applied orthogonal to the M directionwill generate a large torque, γ(M×H_(eff)). As a result, M is excitedinto precession around the effective field direction. The first term inthe LLG equation is the precession term, which describes the spinprecession, and the second term is the energy dissipation term thatdictates the rate of energy dissipation. Adapting spin precessionalswitching promises high-energy efficiency as well as an ultrafastswitching speed.

One of the most important natures in precessional switching of theinvention is that the precession frequency f depends on the effectivemagnetization and effective applied magnetic field. It can be expressedas f²=(γ/2π)² (H_(eff)+M_(eff))+H_(eff), where γ=28 GHz/T. This is alsoknown in the art as the Kittel formula. The equation leads to the factthat it is possible to manipulate the spin precession frequency bycontrolling the effective magnetization and effective field experiencedby the spin.

Accordingly, a writing method according to an exemplary embodiment ofthe invention, for a magnetic memory comprises applying a first magneticfield below a switching field of a pair-cell along the long axis (a bitline) of the memory cells; and applying a second magnetic fieldorthogonal to the first magnetic field, the second magnetic field havinga strength sufficient (for example, stronger than the first magneticfield) to create a spin precession angle large enough to switchmagnetization of one memory cell of the pair-cell to an oppositedirection but not enough strength to switch the magnetization of theother cell of the pair-cell.

The second magnetic field is turned-off as soon as the magnetization ofthe one cell has precessed to more than about ½ but less than about ¾ ofthe precession cycle. For example, when a spin excitation field (e.g Hy)is applied, the magnetization begin to spin from −1 to 0 to 1 to 0 to−1, etc. Therefore, to get the magnetization to stop at 1, the appliedfield needs to be turned-off when the magnetization has crossed the zeropoint and is in the 0 to 1 section. The magnetization cannot just stopanywhere, it has only two possibilities, 1 or −1.

In exemplary embodiments of the invention, the first magnetic field iskept constant for a certain time duration (>0.5 ns) after the secondmagnetic field is turned-off, or in other words, the magnetic fieldduration of Hy is always shorter than Hx.

In this exemplary embodiment, the first magnetic field is applied alongthe horizontal axis and is used to define the final magnetization stateof the cell intended for switching. The second magnetic field isreferred to as the spin precessional excitation field (along the wordline).

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects of the present invention will become moreapparent by describing in detail exemplary embodiments thereof withreference to the attached drawings in which:

FIG. 1 schematically illustrates a multibit device according to anexemplary embodiment of the present invention including two magneticmemory cells (a pair-cell) at one intersection point between the wordand bit current lines in the MRAM architecture;

FIG. 2 depicts creating a non-parallel magnetization in the pair cellsusing shape and/or non-parallel cells alignment;

FIG. 3 illustrates a writing procedure according to an exemplaryembodiment of the present invention in which the magnetization of themagnetic element is initially pointed to the right +X direction andexternal fields are applied to switch the magnetization of the magneticelement to the −X direction;

FIG. 4 illustrates an independent cell addressing procedure of a pairmagnetic memory cells;

FIG. 5A schematically shows an initialization of the pair cells in +xdirection by first applying (+Hy)+(+Hx) followed by (−Hy)+(+Hx); andFIG. 5B depicts a simulation of the multi-bit MRAM pair-cell usingFinite Element Micromagnetic simulation;

FIG. 6 is a schematic drawing showing a four-bit cell structureaccording to an exemplary embodiment of the present invention;

FIG. 7 schematically illustrates a top and cross-section view of aneight-bit cell according to an exemplary embodiment of the presentinvention;

FIGS. 8A and 8B illustrate a stage of forming a two-bit cell structureaccording to the invention prior to an etching process to create thebits;

FIGS. 9A and 9B illustrate a stage of forming a two-bit cell structureaccording to the invention after an etching process to create the bits;and

FIG. 10 illustrates a basic MRAM cell structure with one memory celllocated at the intersection between Word and Bit lines.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE PRESENT INVENTION

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail with reference to the accompanying drawings.

Aspects of the present invention, and methods for achieving them will beapparent to those skilled in the art from the detailed description ofthe exemplary embodiments together with the accompanying drawings.However, the scope of the present invention is not limited to theexemplary embodiments disclosed in the specification, and the presentinvention can be realized in various types. The described exemplaryembodiments are presented only for completely disclosing the presentinvention and helping those skilled in the art to completely understandthe scope of the present invention, and the present invention is definedonly by the scope of the claims.

In a first exemplary embodiment of the invention, a multi-bit MRAM iscreated by a pair of spin-excitation-field-polarity dependent memorycells. The pair of memory cells (a pair-cell) react differentlyaccording to an applied excitation magnetic field polarity.

The first exemplary embodiment includes a memory cell shape which hasits effective remnant magnetization direction slightly tilted off theeasy-axis or long-axis of the cell element.

The remnant magnetization in the pair-cell is tilted in the oppositedirection from the easy axis. This feature allows manipulation of theirmagnetization independently. Therefore, two bits of information can bestored at one cross-point in the MRAM architecture, accordingly reducingthe number of bit and word lines used for the reading and writingprocesses. As a direct result, a higher magnetic memory density ispossible.

Storing two bits of information at one cross-point is possible sincewriting or changing the magnetization of the free layer of one cell canbe accomplished without affecting the information already stored in theother cell.

FIG. 1 schematically shows the first exemplary embodiment of the presentinvention. The multi-bit MRAM 1 includes magnetic and non-magneticmultilayers.

The multibit MRAM includes two magnetic memory cells 2, 3 (a pair-cell)at one intersection point between the word and bit current lines in theMRAM architecture. In the exemplary embodiments of the invention, thememory cells may share the same orthogonal current line (i.e., the samebit and word lines). Further, the memory cells may have a shared orseparate transistor for the sensing operation.

Each of the magnetic memory cells 2, 3, in its most basic form,comprises two ferromagnetic magnetic layers 4, 5, where one the layers 4acts as a free layer in which magnetization orientation can be alteredby an applied external field, while the other magnetic layer 5 serves asa reference layer in which the magnetization direction is fixed orpinned by an antiferromagnetic layer 6 via an exchange bias mechanism.Layer 5 is referred to as the pinned layer.

A non-magnetic spacer layer 7 is accommodated between the ferromagneticmagnetic layers 4, 5. The non-magnetic spacer layer 7 may be either aconductive layer or a nonconductive layer. In the case for a conductivespacer layer, the memory device forms a Giant Magnetoresistance (GMR)structure. On the other hand, a non-conductive layer leads to a MTJstructure.

In either case, the electrical resistance of the multi-bit MRAMaccording to the invention depends on the relative orientation ofmagnetization in both the free magnetic layer 4 and the fixed or pinnedmagnetic layer 5. When the magnetization of these two magnetic layers 4,5 is in parallel, a low electrical resistant state is generated.Conversely, the resultant resistance is high when the two magneticlayers 4, 5 have an antiparallel alignment. These two states ofresistance form the basis of two memories states, 1 and 0.

During a write process, two electrical currents are applied, one alongthe bit line and the other along the word line. Thus, two orthogonalmagnetic fields are generated. The two memory cells can either have aseparated CMOS or a shared CMOS for the sensing/writing procedure.

The magnetic layer 4 is the data layer where information is stored, andfrom which information is read. Magnetization of the layer 4 can bechanged via an externally applied magnetic field and/or a spin-polarizedelectrical current.

Of course, a four-bit cell will have two free magnetic layers 4 percell. In this case, the free magnetic layers 4 may also have distinctmagnetic and physical properties such as saturation magnetization,magnetic anisotropy, layer thickness, magnetostriction coefficient,coercivity, damping constant, and materials, among others.

The magnetically changeable ferromagnetic layers 4 may include the sameor differing ferromagnetic material, as well as a same or differingthickness. In exemplary embodiments, the material may comprise an alloyof Ni, Fe, and Co, or a combination thereof. Further, the ferromagneticmaterial of one memory cell may be different from the other cell,allowing for more efficient spin precession.

FIG. 2 illustrates the shape of cells of the multi-bit MRAM according toan exemplary embodiment of the invention. In the exemplary embodiment,the multi-bit MRAM is designed such that its effective magnetization isnot exactly along the long-axis of the element, but slightly tiltedclockwise or counter-clockwise, due to shape magnetic anisotropy.

This tilting is caused by the demagnetizing field created by the cell,which always keep the magnetization orientation parallel to a flatsurface. At the edge of the cell, the magnetization orientation keepsits orientation parallel to the surface. As a result, this creates a netor effective magnetization orientation along the cell's diagonal axis.

Accordingly, when a fast orthogonal magnetic field pulse is applied toinitiate magnetic spin precession, the precession cone is differentbetween the positive and negative excitation field. Therefore, themagnetization direction in this magnetic element can only be switched byone excitation field polarity but not the opposite below a certainswitching field threshold, hence offering selective writing. Forexample, suppose a magnetic field pulse is applied with a field strengthbelow the threshold value for selective writing. If the Hy is above thisthreshold value, both magnetic elements will be switched together.

As shown in FIG. 2, a pair-cell 20 includes two memory cells 21, 22which have the same shape. However, differing cell shapes may also beused among the cells. As shown, the memory cells 21, 22 have a shapesuch that shorter ends of each cell are at angles that are notperpendicular to the long-axis of the cell ω₁ and ω₂≠90°).

In the pair-cell 20 shown, each cell 21, 22 has shorter edges tilted atangles ω₁ and ω₂ in opposing directions from corresponding shorter edgesof the other cell. Further, one cell 21 has its shorter edges tiltedabove a horizontal plane while the other cell 22 has its shorter edgestilted below the horizontal plane.

The tilting angles of the edges ω₁ and ω₂ are less than 90°,respectively, and in exemplary embodiments of the invention, in a rangeof about 5° to 85°. In addition, these angles ω₁ and ω₂ may be differentamong the cells. The pointed end of the cells (the edges) may also beslightly rounded, as shown, to allow for suppression of magnetic domainnucleation. Of course, cells with non-rounded or sharp edges will workas well.

Further, in the exemplary embodiment depicted in FIG. 2, although cells21, 22 have the same shape, they may be fabricated such that their longaxis are not in parallel, but form an angle θ with the horizontal axis.In exemplary embodiments, the angle θ is in a range of about 1° to 85°.The cell shapes may include, for example, rectangular and ellipticalshapes.

Further still, in another exemplary embodiment, the cells 21, 22 may beconfigured such that one cell forms an angle θ with the horizontal axis,and the other cell includes a tilting angle ω of an edge. That is, theconfigurations of the cells may include a combination of features of theaforementioned θ and ω among the cells.

In an exemplary embodiment, the aspect ratio of the cells is between 1.1and 3.0, and the cells may have the same or differing aspect ratios.

When using pair-cells, it becomes advantageous to sufficiently separatethe cells so that the magnetostatic coupling between cells is minimized.In the exemplary embodiment shown in FIG. 2, the separation distance dbetween cells 21, 22 is ½ W, where W represents a width of a cell. Inthis case, the separation should be greater than ½ W.

A writing method according to an exemplary embodiment of the inventionis explained using FIGS. 3 and 4.

In FIG. 3, the magnetization of a magnetic element is initially pointedto the right +X direction and external fields are applied to switch itto the −x direction. First, a weak −x direction magnetic field isapplied which is followed by a stronger and fast −y direction excitationfield. However, in this case, due to the smaller precession cone (e.g.,where the precession does not crosses zero to the other half of thecycle), the magnetization direction is not changed.

Further, the magnetization of the magnetic element initially pointed inthe +x direction may be changed when a y-axis excitation field isapplied in the positive direction. Accordingly, selective switching isaccommodated.

FIG. 4 depicts an independent cell writing (addressing) procedure of apair of magnetic memory cells according to an exemplary embodiment ofthe invention. The memory cells of the pair-cell, labeled as cell A andcell B, initially have their magnetization oriented in the +x direction.

In FIG. 4, when the y-axis excitation field is applied in the negativedirection, cell A is switched to the −x direction while leaving cell Bunchanged.

Similarly, as shown, if the y-axis excitation field is applied in thepositive direction, then cell B will be switched to the negativedirection while cell A remains in its original direction. Hence, themagnetic memory cell of the invention may be excitation-field-polaritydependent. Accordingly, this behavior leads to allowing a single-cellselection in the multi-bit cell design of the present invention.

In the writing method of the exemplary embodiment, the second appliedmagnetic field (the spin-precessional excitation field) may beturned-off as soon as the magnetization of the one cell, for example,cell B, has precessed to more than ½ but less than ¾ of the precessioncycle. Subsequently, the first applied magnetic field (the easy axisfield) is turned-off after the second applied magnetic field isturned-off. Likewise, the pulse of the easy axis field should have alonger duration than a pulse of the spin-precessional excitation field.In an exemplary embodiment, the pulse duration of the easy axis field isin a range of about 0.2 to 10 ns, while a pulse duration of thespin-precessional excitation field may be in a range of about 0.01 to 5ns.

The selectivity of writing of the multibit MRAM of the present inventionin exemplary embodiments is based on the physics that the effectivefield generated by Hx and Hy exert a different torque on the magneticmoment of the pairs cells. The torque required to reverse themagnetization direction is maximum when Heff is perpendicular to themagnetization direction and minimum when antiparallel.

In addition, the memory cells of the invention may have the same ordiffering spin precession frequencies. Also the memory cells may havethe same or differing magnetic damping constants to provide for betterenergy dissipation, as needed.

FIG. 5A schematically shows an initialization of the pair cells in +xdirection by first applying (+Hy)+(+Hx) followed by (−Hy)+(+Hx).

FIG. 5B depicts a chart created using 3D Finite Element Micromagneticsimulation software to simulate a pair-cell according to an exemplaryembodiment of the invention.

As shown in FIG. 5B, a simulation result is obtained for Mx againsttime. The dotted line shows the timing of Hy. The normalized Mx is zerowhen both cells are switched because the magnetization of the fixedlayer is still in +x. The magnetization in the x, y and z-direction (Mx,My and Mz) may be calculated as a function of time by solving theLandau-Liftshitz-Gilbert equation for the pair of memory cells.

Initially, both cells have positive Mx, thus average unitymagnetization. It can be seen that when a positive excitation field isapplied (t=1 ns), that Mx decreases to half. This indicates that the xcomponent of magnetization of the pair cell is in antiparallelalignment, hence half net magnetization.

At t=3.5 ns, a second negative excitation field is applied. Thisswitches the remaining cell to the −x direction, hence net magnetizationis zero. In this case, both memory cells have negative x-componentmagnetization. In FIG. 5B, the thick, solid line shows the totalmagnetization evolution with time and the thin dotted line shows theapplied Hy.

Of course, the present invention is not limited to a two-bit memorystructure. For example, a four-bit cell structure is shown in FIG. 6.

The bottom free magnetic layers of this four-bit embodiment are labeled“Bottom A” and “Bottom B” while the top free magnetic layers are labeled“Top A” and “Top B”. These four magnetic layers are able to store onebit of binary information each and it is possible to address each bitindependently. This may be achieved by using different ferromagneticmaterial for the bottom and top free layers. This structure gives atotal of 8 logic states.

A writing procedure for this four bit structure is similar to the methoddescribed above where the magnetic memory cells may beexcitation-field-polarity dependent. Table 1 below shows a writingmethod for each magnetic configuration state using a 4 bit cell whichcan include sixteen (16) combinations for writing.

In Table 1, Hx_th is the Hx field above the threshold field to switchboth layers together, and Hx is below the threshold and can only switchone layer. The Top1 and Bottom1 are written by +Hy and the Top2 andBottom2 are written by −Hy. Further, the Bottom layer be themagnetically harder layer. Additionally, in the writing method, +Hxwrites 1 and −Hx writes 0. In Table 1, the contents of the squarebracket correspond to [Top1, Bottom1, Top2, Bottom2].

TABLE 1 +Hx_th and ±Hy [1, 1, 1, 1] −Hx_th and +Hy [0, 0, 1, 1] −Hx and−Hy [0, 0, 0, 1] +Hx and +Hy [1, 0, 0, 1] +Hx and +Hy [1, 0, 1, 1] −Hxand −Hy [1, 0, 0, 1] −Hx_th and −Hy [1, 1, 0, 0] +Hx and −Hy [1, 1, 1,0] −Hx and +Hy [0, 1, 1, 0] −Hx and +Hy [0, 1, 0, 0] +Hx and −Hy [0, 1,1, 0] −Hx and +Hy [0, 1, 1, 1] −Hx and −Hy [0, 1, 0, 1] −Hx and −Hy [1,1, 0, 1] −Hx and +Hy [0, 1, 0, 1] −Hx_th and ±Hy [0, 0, 0, 0] +Hx_th and−Hy [0, 0, 1, 1] −Hx and −Hy [0, 0, 0, 1] +Hx and +Hy [1, 0, 0, 1] +Hxand +Hy [1, 0, 1, 1] −Hx and −Hy [1, 0, 0, 1] +Hx_th and +Hy [1, 1, 0,0] −Hx and +Hy [0, 1, 0, 0] +Hx and −Hy [0, 1, 1, 0] +Hx and −Hy [1, 1,1, 0] −Hx and +Hy [0, 1, 1, 0] +Hx and +Hy [1, 0, 0, 0] +Hx and −Hy [1,0, 1, 0] +Hx and −Hy [0, 0, 1, 0] +Hx and +Hy [1, 0, 1, 0]

FIG. 7 depicts a top and cross-sectional view showing the design of aneight-bit cell structure according to an exemplary embodiment of thepresent invention. Using the four-bit cell structure as shown in FIG. 6as a basic cell, then two ‘four-bit cells’ are stacked on top of eachother. An orthogonal configuration is obtained as an eight-bitstructure. In this configuration, the spin precession excitation fieldfor bits 5, 6, 7 and 8 is applied along the x-axis, and along the y-axisfor bits 1, 2, 3 and 4. All four ferromagnetic layers may have the sameor different ferromagnetic materials.

Multi-bit MRAM of the present invention may be created by growingmagnetic multilayers with distinct, saturation magnetization freemagnetic layers, where each free layer can store one binary bitinformation. The memory cell shape can be, for example, rectangular orelliptical. The layers may be formed by film-patterning, doping,etching, and polishing techniques known in the art.

As shown in FIGS. 8A and 8B, during the manufacturing process, sourcearea 84 is formed to the left of gate 82 while common drain area 86 isformed to the right of gate 82. These form the basic of a transistorstructure. Magnetic multilayer 850 as described in [58] [59] is formedto cover the first and second pads 800, 806. Also formed are first andsecond patterns pr1, pr2 above a predetermined area of layer 850.Because patterns pr1, pr2 are areas where the actual bit data is to berecorded, pr1, pr2 are formed closely, but do not contact each other.The distance between pr1 and pr2 may be adjusted during themanufacturing process, as well as the shape of pr1 and pr2. Then, pr1and pr2 are used as masks to form the memory bits from layer 850.

FIG. 9A shows the memory bits 90 a and 90 b that have been formed fromlayers 850 using pr1 and pr2 as the masks through multiple steps ofchemical and/or plasma etching processes. The etching process is stoppedonce layer 806 is exposed. The remaining portions of pr1 and pr2 must beremoved through further chemical and/or plasma cleaning processes.Subsequently, as shown in FIG. 9B, layer 98 is formed to cover thememory bits 90 a and 90 b and first and second pads 800, 806. Layer 98may be smoothed until the memory bits 90 a, 90 b are exposed. Methodsfor smoothing layer 98 may include, for example, etching andchemical/mechanical polishing. Layer 98 is non-conductive and itsfunction is to isolate the first and second contact pad 806. The topcontact pad will be deposited at the top surface and in contact withmemory bits 90 a and 90 b.

The difference in saturation magnetization allows recording informationin one bit without affecting the information in the other bit. Thisallows for selective writing, and is made possible because the twomagnetic free layers have different spin precession cone angles.

Accordingly, the present invention allows for improved data density byusing a multi-bit MRAM structure, where the memory cells are arrangedsuch that an effective remnant magnetization of each of the cells isnon-parallel from the cells' long axis.

Further, the present invention may use spin precessional switching whichis energy efficient.

In addition, high frequency cell writing is possible, therefore leadingto a higher data transfer rate.

Finally, the present invention allows for reducing the number of bit andword lines in the current MRAM architecture for a multi-bit MRAM.

Although exemplary embodiments of the present invention have beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims, including the full scope ofequivalents thereof.

1. A writing method for a magnetic memory comprising at least onepair-cell having two cells, the method comprising: creating torque onthe cells of the pair-cell by manipulating at least one of themagnetization and the effective field angle of the cells such that atorque applied to the cell to be switched is larger than a torqueapplied to the other cell, wherein the torque applied to the cell to beswitched has a strength sufficient to create a spin precession anglelarge enough to switch magnetization of the cell to be switched to anopposite direction but not enough strength to switch the magnetizationof the other cell.